Studies are being carried out on the molecular mechanism of the excision repair of UV radiation damage to DNA. Both a pyrimidine dimerspecific endonuclease and an exonuclease activity that excises dimers have been identified in extracts of phage T4-infected cells. The endonuclease has been extensively purified and characterized. Enzymologic studies are also being done with cell-free extracts of human cells, with a view to isolation and characterization of the human excision repair enzymes and comparisons between normal and xeroderma pigmentosum cells. The role of DNA repair and its defectiveness in carcinogenesis is being examined in xeroderma cells by investigating somatic mutation frequency in these cells after DNA damage.